There are several Constantly Variable Transmissions (CVTs) that are being successfully designed and manufactured as transmissions for vehicles and other machines that require changing gear ratios.
The market for these drives within the automotive industry, at least, is generally split between belt drives and traction drives. The present invention is related to traction drives.
The traction drive market is dominated by two similar types of mechanism. One is generally called a Half Toroidal drive and the other a Full Toroidal drive.
Both of these involve the use of twin discs that are machined with the negative shape of a toroid in opposing faces. Between the discs are rollers that roll against the surface of both toroidal cavities and can transfer force and power from one disc to the other. Both of these drives use a special fluid called a traction fluid to allow a force transfer from the rollers to the discs without requiring metal to metal contact.
These rollers can be rotated so that they contact the discs in different places and in so doing can change gear ratio in a seamless manner as they move.
In a “Full Toroidal” drive the rollers centre of rotation is located on the centre of the toroidal cavity. In a “Half Toroidal” drive the rollers are located off the centre of the toroid towards the centre of rotation of the toroid.
Both types have advantages and disadvantages over each other and a tradeoff must be made in both cases using geometry to take advantage of the advantages and lessen the effects of the disadvantages.
An example of a typical Full Toroidal Variator is disclosed in U.S. Pat. No. 5,989,150, the disclosure of which is incorporated herein by way of reference. Two toroidal cavities are created between four discs which are clamped over rollers running on ball bearings and located generally in the centre of the toroidal cavity formed between the discs. The clamping system uses a roller cam arrangement located in the lower part of the diagram and a spring washer that provides a minimum clamp when no torque is being transferred or when changing from an acceleration mode to a deceleration mode. U.S. Pat. No. 5,111,710 discloses a typical toroidal cavity which contains three rollers.
An example of a Half Toroidal Variator is disclosed in U.S. Pat. No. 4,934,206. The disclosed Half Toroidal Variator exhibits the same twin cavity arrangement as in U.S. Pat. No. 5,989,150 and a similar clamping arrangement. U.S. Pat. No. 5,048,359 discloses a typical half toroidal cavity having three rollers mounted off-centre to the toroidal cavity.
It can be seen that in both cases a double toroidal cavity is used with each cavity typically containing three rollers. This configuration is adopted so that there is no need to support the clamping force on a rotating bearing. The clamping force is provided by the two outer toroidal discs which typically perform the function of the input. This necessitates the output being taken off via a secondary lay-shaft displaced from and running parallel to the main disc rotational axes.
Because the input and output in a typical single roller design rotate in opposite directions the differential speeds of the input and output are very great. The clamping force required to sustain the traction forces are also very high. Without the use of the double cavity the energy loss associated with supporting the rotational loads using bearings it considered too high by most designers.
The maximum practical number of rollers in the Half and Full Toroidal drives is three after which it becomes impossible to maintain sufficient ratio change. Typical ratio changes or ratio spread for both types is 6:1 although 7:1 is possible with the Full Toroid.
Considerable advances in the formulation of traction fluids have been made associated with the introduction of traction drives to the general market. They have unique property that causes them to increase in viscosity by billions of times when subjected to pressures of typically above 1 GPa and momentarily almost solidify when under high pressure. In a traction drive this solidification effect occurs as one surface rolls over the other and allows the forces to transfer from one mechanical part to the other through the fluid not via direct contact of the parts themselves, protecting the metal parts from damage. U.S. Pat. No. 4,392,973 describes a formulation for preparing a traction fluid using borate ester formulated in a particular way. The patent claims the use of the particular traction fluid in traction devices for transferring power.
Generally speaking the Full Toroid is simpler than the Half Toroid and has no heavily loaded thrust bearings, because most forces are balanced within the toroids themselves. It is also simpler to control and can employ what is referred to as torque control which is a very stable and reliable form of control. Torque control relies on balancing the reaction forces exerted by the rollers on a piston supplied with oil at a controlled pressure. The piston is arranged to move in a direction slightly off line to the plane of the discs called the “castor” angle and as it moves the roller which is connected to the piston via a ball joint is forced to steer to a new ratio in a controlled manner. Examples of such control are disclosed in U.S. Pat. Nos. 59,891,500, 7,160,226 and 4,186,616, the disclosures of which are herein incorporated by way of reference.
However the Full Toroidal Variator suffers from a mechanical problem called “spin”. On the other hand the Half Toroid must deal with heavily loaded thrust bearings but does not suffer as much from “spin” and so the contact points can be larger, and when in one particular position can be designed to suffer no spin at all, where the surface velocities of the rollers and the toroidal discs almost perfectly match at the contacting points.
Considerable efforts have been made over the years to combine the control methods used in the Full Toroid with the low spin characteristics of the Half Toroid. U.S. Pat. No. 5,895,337 describes one such method.
“Spin” is a term often used when describing traction drives and refers to the spinning effect that happens when two surfaces roll over each other in such a way that the surface velocities do not exactly match each other. It can be likened to the “scrubbing” effect of a cylindrical roller continuously turning a corner.
In order to appreciate this problem refer to FIG. 1 which diagrammatically represents how these differential velocities come about. The variator main component parts consists of Input discs 100 and 101 driven by the input shaft 108.
Output discs 102 and 103. Rollers 106 supported on axles 104 running on bearings 105. A clamping plate 109 rigidly fixed to the shaft 108 and bears onto the upper disc 100 with hydraulic cylinders 107 to provide a clamping force. The disc 100 can move axially up and down on the shaft 108 but cannot rotate.
When the Variator is rotating it can be seen that the surface of the roller which is essentially cylindrical in shape will adopt a uniform speed across its width of R1×its angular velocity. However the corresponding speed on the input rollers will vary between R4×the input angular velocity and R5×the input angular velocity. The roller is forced to adopt a compromise speed somewhere between the two speeds on the discs. Only one infinitely narrow band of contact has equal velocity. A similar situation exists for the output contact patch because of a similar difference in velocities associated with the different radii R2 and R3. Nevertheless the variation in velocity of the contact surfaces can be maintained without damaging the metal parts because of the traction fluid's unique ability to shear without damage, up to a limiting speed (and pressure) where so much heat is developed that the traction fluid begins to break down or the hardened metal surfaces begin to anneal and soften.
When two surfaces roll over each other so that there is no difference in surface velocities then the traction fluid works well and can maintain quite high apparent traction coefficients of up to 0.1. When relatively high spin occurs the traction coefficient may maintain but a high degree of slip (loss of efficiency) occurs creating an “apparent” lowering of the traction coefficient. When the spin exceeds certain limits the high temperatures created under the patch will damage the traction fluid. These high temperatures can also affect the hardness of the steel limiting the types of steel and types of heat treatment process used to harden the components.
Many inventors have applied their talents to reducing the problems associated with overheating of contact patches including U.S. Pat. No. 7,211,024 that describes a method of spraying the roller with pre-cooled traction fluid, adopting mechanical enclosures to ensure the roller is washed in oil and using radiation to radiate heat away from the roller. US patent application publication no. 2007/0,204,940 describes a method of improving the properties, hardening methods, and surface treatment to enable the rollers to withstand higher temperatures. Attempts have also been made to reduce the overall size of the device by simplifying the method of roller control such as described in US patent application publication no. 2008/0,254,933.
The mechanical load transfer of a traction drive is completely different than the mechanical load transfer on friction gears such as railway wheels, where virtually no slip is allowed to occur as this would damage the wheels and rails. However prior to the advent of traction drives, made possible because of the development of traction fluids, there were many friction drives including Full and Half Toroidal Friction drives that utilized dry running friction to transfer power. Because of the spin problems described earlier associated with Full Toroidal Friction drives, although they could support high power and were relatively efficient, they were subject to fatigue and the moving parts needed replacement often. U.S. Pat. No. 2,595,367 (Picanol) proposed a method of at least partly solving this problem of fatigue, by replacing a single roller with double rollers, the disclosure of which is incorporated herein by way of reference.
The Picanol system does not appear to have been seriously implemented in any commercially available transmission. It is perceived that the Picanol system would suffer from mechanical difficulties associated with mounting of the rollers.
French patent no. 7961882 describes a method of applying a double roller concept to a Half Toroidal traction drive type.
The Picanol system was designed to solve a problem of fatigue that badly affected early friction drives. Its design appears to have been based on the fact that the coefficient of friction of dry steel on steel or steel running in a low viscosity oil is greater than 0.3. Although there are no calculations associated with the disclosure it can be understood by someone skilled in the art of traction drives that the clamping arrangement is very light and would not be capable of clamping sufficiently to enable an effective power transfer when using traction oil with a traction coefficient (friction coefficient) of around 0.05.
The Picanol system differed from the earlier French invention in that the rollers used two separate contacting surfaces. One to contact the toroidal disc and one to contact the other roller. The French patent used the Half Toroidal configuration described earlier while the Picanol patent used the Full Toroidal configuration.
The Picanol system arranged two rollers within the toroidal cavity in place of the single roller. By angling their rotational axes it was possible to closely match the surface velocities of the rollers and the discs and so avoid the problem of spin.
It is with the Full Toroidal Traction Drive concept that the present invention is specifically concerned. In this regard it is specifically concerned with the replacement of the Single Roller with a Double Roller.
The Picanol system was designed to solve a different problem than the current invention is concerned with. It set out to reduce the differential velocities of the contacting patches so that the friction drive or gears as they were referred to in 1947 were subject to less wear. Because the dry friction coefficient associated with Friction Drives is so much higher than the “wet” friction associated with Traction Drives the clamping forces applied in the Picanol system were small and the associated support systems light.
A typical Full Toroidal Variator can absorb 10-20 Nm of torque per liter of mechanical volume. The Picanol system appears to be designed to absorb a similar level when running as a friction drive but if running in a traction fluid could absorb only 25% of this.
The double roller as described in Picanol did describe a way in which the durability of a full toroidal variable friction gear may be improved however it did not describe a way in which the more modern traction drive running in traction fluid could be improved in power density. If such a drive was converted to a traction drive, problems would be encountered with most of its mechanical arrangement if any attempt to increase the power density beyond the current level.
Picanol required that the rollers be supported on a single “V” shaped rigid dual axle support. This support confirmed that the rotational axes of the rollers were set at the angles that ensured that the surface speeds between discs and rollers at the contact points remained as similar as possible.
In order to increase power densities of a traction drive where the frictional coefficient or more appropriately the traction coefficient is typically only 20% of the dry friction coefficient, it is necessary to increase the clamping force by several times. Without such increases the double roller design will remain less power dense than the much simpler single roller design.
When the clamping pressures are increased the roller support mechanism involving two connected axles described in Picanol become unworkable.
When the conical sections of the rollers are subject to pressure on their mating surfaces very small deflections occur. The size of these deflections varies with how much of the load they are carrying and the variable radii of the cones themselves and the structure of the rollers themselves. When heavily clamped, the rollers will attempt to adopt positions where the included angle of their rotational axes is not exactly equal to the unloaded included angle between the cones. In a working mechanism this will load up the supporting bearings which will attempt to resist the deflected position. When the clamping forces are high and the width of the conical contacts large in proportion, this will at best lose efficiency and at worst very rapidly collapse the bearings.
Similarly the deflection on the conical roller surfaces will reduce the distance between the roller axles. Unless the “V” shaped connected axles can displace themselves laterally within the plane that contains them this will load up the bearings and inhibit the force carried by the conical surfaces necessary to transfer load, or damage the bearings, or bend the axles.
It may be possible to design a way around this by carefully studying the true angle of the clamped cones and adjusting the angle included by the supporting axles accordingly. However in modern traction drives the clamping force is typically adjusted up and down dependant on how much power or torque is passing through the machine. Any fixed angle will immediately only work in one clamping state. In other positions the same problem will exist.
It is an object of the present invention to adopt the double roller design of Picanol for use in a traction drive having developments which eliminate the roller support problems present in Picanol. Further, such a double roller design can exhibit much higher power densities than a Single Roller Full Toroidal Drive, and can be controlled in a manner similar to modern control philosophy and with durability expectations consistent with modern traction drives.